Introduction to Mechanical Power Transmission
Types of Mechanical Power Transmissions
In Mechanical Engineering, Power transmission is harnessing the mechanical elements and put them at work. WORK is to perform and succeed, producing good results. Power transmission elements can vary from Screws, Wedge, Pulleys, Belts, Cam, Gear, Gear box, Walking beam, Hoppers, Pneumatics and hydraulics, Motors, and more…. Depending on application and design where applied physics governs major concept. People, Design Engineers harness these physics theory and put them all together in order to perform a WORK.
Screws for example have significant output to work. It can lock, and assemble parts using the principles of inclined plane that is put into helix. It can lift loads too, transport using the auger conveyor principle and some applications of un-imaginable magnitude. Thousands of application can be found in the Machineries or in the manufacturing plant. The work performed by this wonderful invention has served mankind for several centuries now and are still in continuous development for special Applications like precision adjustment, surgical implants, robotics and automation.
Wedges, using also the mechanical work advantage of an inclined plane has served mankind from the pre-historic time to the present in many forms; Timber cutting by the lumberjacks; Household firewood cutting; Ancient weaponry; Fire fighting; Even in sports; As well as industrial machineries.
Since the invention of wheel, pulley has emerged and was harnessed for powerful mechanical advantage. Pulley and compound combinations has been developed to transmit and carry heavy loads as normally, 1 pulley can reduce the applied weight by half, and as compounded with another it can carry really heavy loads. Pulleys are also used to transmit power by diametrical pitch ratio. Like in this engine drive that the driven pulley carries the work. Pulleys are used to reduce speed too by the diametrical ratio of the Drive and the Driven pulley. Pulleys are driven conventionally with rope and belts. In precise application, Timing belts or toothed belts are employed. Belts and Chain links are the common medium of power transmission. It serves the purpose of isolation over-drive safety.
The Pulley and Belts:
Belts can be; Rope; Reinforced fiber rubber; “Vee” belts; Flat belts; and Toothed belts (timing devices)
Width of “V” belts must be the same as the pulley width or slightly wider. Depth of pulley groove must be at least 1mm deeper than the belt engagement. “V” Belt is designed to grip from its sides. Belly grip is not allowed as it will potentially slip from the pulley.
Flat belts are designed to grip on its belly. This application however allows the belt to slip if the grip force is overcome. Grooved flat belt gains more surface area for greater force.
Flexible power transmitting elements for conveying torque and angular displacement between non-collinear shaft especially suitable for large center distances. Its advantages includes; High mechanical efficiency; Non-slip operation (positive drive); Low maintenance cost; Quiet operation; High initial tension not necessary; Large capacity in small space; Can tolerate low arc of contact (minimum of 6 teeth in contact for rated power); Minimal backlash. It also has disadvantages which includes; susceptible to fatigue failure when used with small pulleys; Transmission of shock loads; Can be used only with mating pulleys .
Belts are selected according to the length of the belt. Timing belts may also be selected according to the number of teeth using the following formula:
– Belt length = number of teeth x pitch
Belts are selected also according to the width of the belt. Timing belts must be selected from the variable standard belt widths.
Belt tensioning basically is to ensure that the driving belt transmit the power to its highest efficiency. Too loose belt can induce belt-slip that losses efficiency and heat, while too tight belt tension losses efficiency due to additional load of the driver. Tension belts to its manufacturers specification, or just right that it will not slip from the pulleys.
Mechanical element used to connect collinear shafts directly to one another with the purpose of transmitting torque and/or revolution angles.
Rigid coupling. Used when shafts are virtually collinear and should remain in a fixed angular relation with respect to each other. Disadvantages includes; true alignment of shafts is difficult to achieve and difficult to maintain; Most successful when used at low speeds and with flexible shafts; Susceptible to fatigue failure and overheating of bearings.
Flexible coupling. Used to take care of inevitable shaft misalignment. Absorbs shock and vibration that may appear on one shaft.
Gear is commonly used to transmit power at rotary or linear motion depending on application. Knowing that gear mesh at sliding clearances, they are governed with clearance and tolerances that will allow them to move freely. With this consideration, Gears are prone to the known phenomena of backlash. Backlash is ability of the component or part to recoil, a reaction to an action or work. If this phenomena is not controlled, the mechanical element will lose its precision. Backlash is measured in arc minutes. Spur gear is a cylindrical gear on which the teeth are cut in parallel to the shaft. It has the disadvantage of noisy meshing teeth. The worm, helical gears are developed to overcome the spur gear weaknesses. Such has high rate of contact that can deliver lower noise and higher strength, but its axial thrust requires high degree of consideration as this will cause separation force of the meshing gears.